The present invention relates to an apparatus used for X-ray diagnosis and, more particularly, to a digital fluorography apparatus which obtains an X-ray image and performs digital image processing of the obtained X-ray image.
A digital fluorography apparatus consists of an X-ray TV apparatus and a video-signal digital processing unit. The X-ray TV apparatus comprises an X-ray tube serving as an X-ray source for generating X-rays to be radiated onto an object; an image intensifier (to be referred to as an "I.I." hereinafter) which faces the X-ray tube and serves as an X-ray/photo converter for converting X-rays transmitted through the object into an optical image; a TV camera for imaging an output image from the I.I.; and a monitor for displaying an image imaged by the TV camera. Operation modes of the digital fluorography apparatus include a fluorography mode for observing a fluorographic image of an object inserted between the X-ray tube and the I.I. by means of a TV image displayed on the monitor, and an imaging mode for recording the fluorographic image of the object and performing digital image processing, if necessary. Therefore, in the digital fluorography apparatus, the desired portion of an object to be imaged is found upon fluorographic observation in the fluorography mode, and the imaging operation and image processing are performed when the operation mode is switched to the imaging mode.
Recent digital fluorography apparatuses have adopted a digital subtraction imaging method for extracting an image of a contrasted portion of an object through an arithmetic operation during, e.g., angiography (an imaging method for obtaining an X-ray image by injecting a contrast medium in a blood vessel). In this imaging method, image (mask image) data of a portion of interest before contrasting (before injection of a contrast medium) is subtracted from image data after contrasting (after injection of the contrast medium) so as to obtain, as an image, a difference between X-ray absorption values due to the presence/absence of the contrast medium. When the mask-image subtraction processing is performed with respect to a plurality of subsequent frames of a video signal after injection of the contrast medium, a plurality of image frames representing sequential flow of the contrast medium can be obtained. When a plurality of image frames are sequentially displayed, contrast medium flow can be observed as an animation. In order to obtain a processed image which can provide a high diagnostic effect with the above image processing method, the amount of light incident on the TV camera, which corresponds to an X-ray image converted into an optical image by the I.I., must be controlled to fall within the dynamic range of the TV camera.
In the digital subtraction imaging method, however, if an X-ray dose to the object is changed during imaging, an unnecessary image (an image indicating a change in dose and/or an image indicating a change in absorption characteristics due to the change in dose) caused by the change in dose appears during subtraction processing, resulting in an image having a poor diagnostic effect. For this reason, an X-ray dose to the object must be set at an optimal value before imaging takes place.